rangelock: Fix an off-by-one error

A rangelock entry covers the range [start, end), so entries e1 and e2
with e1->end == e2->start do not overlap.

PR: 281073
Fixes: 5badbeeaf061 ("Re-implement rangelocks part 2")
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46458